1. Field of the Invention
This invention relates generally to a process for removing contaminants from a gas stream.
More particularly, this invention relates to the removal of volatile organic compounds from air streams.
The build-up of volatile organic compounds in ambient air is of increasing concern as such compounds are now recognized as a major source of air pollution in many urban areas. Many volatile organic compounds are released into the air through the inevitable discharges accompanying industrial processes and chemical manufacture and the effects of such air pollution have had much attention.
It has also become evident that the treatment of municipal liquid wastes releases substantial quantities of volatile organic compounds into the atmosphere. Some of these compounds find their way into collection systems as oils and other wastes which are dumped into sewers and some by run-off from rain washing streets with residues from automotive traffic. The "sanitary" waste water treatment system treats biological wastes and does not deal with many of the hazardous and often toxic volatile organic compounds in municipal sewage. Some of the volatile organic compounds are absorbed by the sludge produced in treatment plants but, when the sludge is composted or dried, the compounds are again released. No satisfactory means for control of these pollutants is presently available.
Municipal waste water treatment facilities liberate volatile organic compounds in a number of different treatment operations. Such operations include, for example, pumping stations, trickling filters, aerobic digesters, aeration basins composting, sludge drying, and the like. The variety and quantity of volatile organic compounds liberated into the atmosphere in such unit operations include many compounds that one would not ordinarily expect to be found in sewage sources. Major classes of organic compounds identified in studies of process air streams in certain municipal waste water treatment plants include hydrocarbons of all sorts; aromatics including benzene, alkyl benzenes, toluene, xylenes, naphthalene and the like; oxygenated compounds such as alcohols, ketones and epoxides; halogenated compounds including chloroform, trichloroethylene, methylene chloride and freons; nitrogenous compounds such as pyridine and various nitriles; and sulfur containing compounds including dimethyl disulfide and mercaptans. The concentration of individual compounds typically ranges, in gaseous emissions, from a few parts per billion to a few hundred parts per million. Because of these low contaminant concentrations and because of the very large volume of air involved, ordinary treatment methods such as direct combustion, adsorption, and the like, are either not applicable or are prohibitively expensive. Conventional technologies using recirculating chemicals concentrate and revolatize pollutants which re-contaminates the exhaust gases.
Many industrial processes liberate enormous quantities of hazardous volatile organic chemicals. A recent survey indicates that in 1987, 237 billion pounds were emitted by industry into the air in this country. A wide variety of compounds were emitted with the major contaminants including toluene, trichloroethane, ammonia, ethylene, xylene, chlorine, methyl ethyl ketone, trichloroethylene, methanol, carbon disulfide and many others. Many commercial operations also liberate volatile organic compounds to the atmosphere. Examples of such polluting operations include auto paint spray shops, dry cleaning establishments, food service facilities, print shops, furniture refinishing operations, and the like.
Attention is also being focused on the air within enclosures including the interiors of processing and manufacturing plants and office buildings. Office buildings, in particular, often display levels of pollutants many times higher than that of the outside air. Recent trends in office building construction include sealed windows and a high level of air recirculation with little exchange of interior and exterior air, in order to maximize energy savings and to gain better control of temperature and air circulation. At the same time, the amount of gaseous pollutants released into the building air has tended to sharply increase. Typical sources of volatile organic compounds released into the atmosphere within a building include emissions from carpets, carpet backings, furniture fabrics and padding, and foamed plastic packing materials as well as solvent emissions from operating and cleaning office equipment and food service operations. In addition to gaseous contaminants, the atmosphere within many buildings contains finely divided solid contaminants including smoke, pollen and air borne bacteria and viruses. In some instances, the combined load of gaseous and particulate contaminants within a building have become so high as to cause allergic reactions and respiratory distress among a large proportion of the building's tenants. The usual treatment or conditioning of recirculating air within buildings and other enclosures is limited to the adjustment of temperature and humidity and particulate removal, usually by filtering. Such conditioning treatments are ineffective for removing gaseous contaminants and often are inadequate to remove fine particle contaminants.
2. Description of the Prior Art
One treatment method proposed in the patent literature for the removal of organic pollutants from air is the set out in the Merrill patent, U.S. Pat. No. 3,593,496. Merrill discloses that organic pollutants such as hydrocarbons can be removed from air by mixing the air with an aerosol of water droplets containing a surfactant which presents an oleophilic surface on the water droplets. The aerosol droplets absorb organic pollutants into and on their oleophilic surfaces and removal of the droplets from the air stream leaves a substantially purified air stream. Merrill prefers to form his droplets from aqueous suspensions of lecithin compounds as the surfactant.
Another process, which has come to be know as mist scrubbing, has recently been developed for removing contaminants, notably odorous contaminants, from gas streams. This process uses an aqueous solution of one or more chemicals which are reactive toward or able to solubilize the odorous contaminants. Contact between the reagent solution and the gas is accomplished by atomizing the aqueous chemical solution into very tiny liquid droplets and dispersing the droplets into the gas stream. The liquid droplets are small enough such that they do not immediately settle out but instead flow with the gas much in the manner of a natural fog. Typical installations utilize droplets having a number median diameter on the order of about ten microns. Mist scrubbing processes are illustrated by U.S. Pat. Nos. 4,125,589 and 4,238,461, both to deVries.
In typical mist scrubbing processes, a suspension of atomized reagent droplets in an air stream is passed in concurrent fashion through a gas-liquid contacting chamber or scrubber vessel. It is usual practice to introduce the reagent droplet suspension into the top of the scrubber vessel and to remove a cleaned gas stream from the bottom of the vessel. The reaction vessel contains no packing or internal media of any kind and is sized to provide the desired reaction time, typically ranging from about three to sixty seconds, between the gas and droplets.
Drain means are ordinarily provided at the bottom of the vessel to remove that spray liquid which settles out in the contacting step and the collected liquid is discharged as a waste stream. Additional points of liquid collection are also provided following the contacting chamber at fans, elbows, stack bottoms and the like. Depending upon reaction conditions, particularly contact time and the size distribution of the spray droplets, the amount of spray liquid which settles out, and is removed from, the vessel is normally less than the amount of liquid introduced into the scrubber vessel in the droplet spray. The remainder of the spray liquid is carried from the reaction chamber with the exiting cleaned gas stream either as a vapor or as a suspension of tiny droplets or is volatilized to saturate the gas stream. A nearly complete reaction between the reagent and the gas contaminants can routinely be achieved. Because of the low liquid flow rate and that the reagents ordinarily used in odor removal are reacted and reduced to low concentrations and comprise chemicals such as sodium hypochlorite, sodium hydroxide and sulfuric acid, the escape of some exhausted reagent droplets in the cleaned air stream is of no significant concern.
However, with the recognition of the presence of hazardous, toxic, and not innocuous components in the gas stream, it became necessary to observe the fate of those compounds as treated by mist scrubbing. It soon became clear that significant uptake of compounds into the liquid droplets was being accomplished even for compounds which were not expected to be significantly soluble or reactable.
A study totally unrelated to gas scrubbing technology and concerning the concentrations of pesticides found in morning mists above agricultural fields suggests that process mechanism analogous to those employed by Merrill and deVries may also occur in the natural environment. Researchers D. E. Glotfelty et al, writing in Nature, Volume 325, Pages 602-605, Feb. 12, 1987, reported that certain natural fogs contained unexpectedly high concentrations of pesticides, herbicides and other chemicals. Fog sampled in Beltsville, MD and in the San Joaquin Valley of California was found to contain concentrations of some toxic substances that was many times higher than was predicted by calculations using Henry's Law. Concentrations of insecticides such as malathion and herbicides such as alachlor in the fog droplets was far higher than was the level of these compounds in the surrounding air.
Their reported data showed that enrichment into the fog droplets was more pronounced for hydrophobic pesticides than for hydrophilic ones. The authors proposed two hypotheses to explain the enrichment. One hypothesis was that the fog droplets contained solutes such as dissolved or colloidal organic material which increased the solubility of hydrophobic compounds thereby shifting the equilibrium to the solution phase. A second hypothesis was based upon the authors' observation that surface-active, non-pesticidal organic matter was present in the fog liquid as shown by its foamy, soapy appearance. Although the authors cautioned that they had no experimental verification, they considered it to be a reasonable conjecture that surface-active material might have been present in sufficient amounts to produce an organic film on the surface of the fog droplets. Thus, the surface-active organic matter presumed to be present at the air-water interface acted to enhance the uptake of pesticides into the aqueous phase in a manner reminiscent of the process described by Merrill.